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Motion amplification of agitator

An agitator installed in petrochemical plant is reported excessive vibration amplitudes measured at motor, gearbox and associated structures when operating speed is increased from 42 to 84 rpm due to process requirement. Motor running speed is about 1488 rpm (or around 24.8 Hz). Vibration amplitudes is maximum at motor non-drive end (NDE) measurement point with amplitude 8.4 mm/s, RMS. Predominant frequency components are at agitator/mixer and motor speed.

Our approach: We want to analyze vibration characteristics of this unit in details before appropriate solution and remedy action can be taken so the following tests were carried out. Also, we would like to cross-check our new tool, Iris M Motion Amplification camera with conventional operating deflection shape (ODS) analysis.

  • Conventional vibration measurement at motor, gearbox, agitator, and associated structures

  • Operating deflection shape analysis

  • Motion amplification analysis with Iris M

  • When machine stopped, experimental modal analysis was performed

  • FEA modal analysis was carried out in parallel in case of structure modification and simulation is required.

Results: Motion amplification data showed well agreement with ODS results as well as vibration data measured. Experimental modal analysis showed that first and second natural frequency are 8.4 and 26.6 Hz respectively and this is close to results of FEA modal analysis. Therefore, high vibration amplitude predominantly at agitator shaft speed is not due to resonant problem but it was suspected excessive shaft runout/eccentricity and weak motor support. However, second mode is quite close with motor running speed with separation margin less than 20% so structure modification is required.

Findings and Solutions: Agitator shaft runout was found 4 mm over specification at 2 mm. Structure modification is recommended to shift second natural frequency away from motor running speed as well as to add more support/stiffness to the motor. Natural frequency of new system was predicted to be 13.45 and 39.49 Hz for first and second respectively. This is considered safe from excitation from motor/agitator running speed as well as its blades pass frequency. Temporary support was added to the motor and vibration amplitude showed significant reduction, hence, permanent support will be added as per our drawing submitted to customer.

With proper tools and diagnostic skills, we can see complete picture of vibration characteristics. Root cause can be correctly identified, hence, solution can be selected and implemented effectively without trial and error. This saves time and money as the end result.

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